The instant invention relates to processes for the manufacture of ethylene oxide. The production of ethylene oxide by the reaction of oxygen or oxygen-containing gases with ethylene in the presence of a silver-containing catalyst at elevated temperature is a key process in the chemical industry. Due to the flammable nature of oxygen, these processes rely on precise and accurate control of oxygen, and particularly, the “Limiting Oxygen Value” (“LOV”), also known as the Maximum Allowable Oxygen Concentration “(MAOC”). The LOV is the oxygen concentration at which a combustion reaction will propagate through ethylene oxide process gas. Those of skill in the art are familiar with formulas for the calculation of LOV. Using too much oxygen can result in a catastrophic ignition, while using too little can result in poor yield. Independent reactor inlet and outlet oxygen analyzers are also used for automatic safety shutdown and isolation of oxygen feeds. If the capability to monitor inlet oxygen concentration continuously is lost, oxygen and hydrocarbon feeds must be immediately shut off. If the capability to monitor outlet oxygen concentration continuously is lost, either (a) the affected reactor must be shut down immediately or (b) the inlet oxygen concentration must be kept below the outlet operating limit. If alternative (b) is chosen, the reactor must be shut down immediately if the inlet oxygen concentration exceeds an offset from the LOV to ensure that a safety margin can be maintained. The size of the offset depends on the system geometry and past history of decompositions. For example, the shut down could be triggered where the oxygen concentration exceeds outlet LOV+1 vol % (based on LOV calculations before loss of capability to monitor). (Many commercial ethylene oxide plants would choose not to operate under this option (b). Therefore, it is very important to control the LOV at the reactors with a high degree of accuracy and precision. In fact, most ethylene oxide facilities demand that analyzer systems and instrumentation have full redundancy.
Currently, the best practice in the industry is to use an oxygen measurement based on a paramagnetic analyzer. Limitations in the measurement itself can dramatically affect the ability to control the LOV at its optimum, and therefore limit the overall efficiency and yield of an ethylene oxide plant. There are several drawbacks to use of paramagnetic analyzers for controlling LOV:
(1) Many non-oxygen components of reaction system gas depress the oxygen concentration indicated by a paramagnetic analyzer, causing a paramagnetic offset. There are two ways to compensate for this offset. The first is to calibrate concentration at the midpoint of the range of the non-target gases. A disadvantage to this approach is that the compensation will be an average of the offset and the uncertainty of the measurement increases. The second is to compensate with “live” input from a gas chromatograph or gas chromatograph/mass spectrometer. Disadvantages to this approach are that the data is not real-time and that the reliability of the mass spectrometer is not as high as using other methods.
(2) Oxygen is reactive. Questions arise concerning sample integrity when the sample is transported through 10-100 meters of tubing to an analyzer in an analyzer shelter, such as with the paramagnetic analyzers.
(3) The process temperatures in the ethylene oxide streams can be as high as 330° C., but the temperature limit for a paramagnetic analyzer is about 130° C. Thus, the sample temperature must be reduced prior to analysis.
(4) The process pressures in the ethylene oxide streams can be as high as 350 psig, whereas the pressure limit for a paramagnetic analyzer is about 50 psig. Thus, the sample pressure must be decreased prior to analysis.
(5) The paramagnetic analyzers become fouled by the solids/liquids in the streams, causing mirrors to coat and cells to short. Thus, the samples can destroy or compromise the measurements.
(6) The paramagnetic analyzers take time to transport the sample to the sheltered analyzer and take additional time to condition the sample (reduce temperature, decrease pressure).
(7) The paramagnetic analyzers cell vents are connected to a cell vent header and require pressure compensation. Variability in the pressure compensation leads to uncertainty in the oxygen measurements.
All of these drawbacks to paramagnetic analyzers result in introducing variability into the control of LOV. Thus, there is a need in the art for a method to more quickly, accurately and precisely measure oxygen in ethylene oxide processes than the best practice in use today.
The development of the tunable near-infrared diode laser and absorption spectroscopy approach for the determination of oxygen, carbon monoxide, and oxides of nitrogen in the combustion gas from a coal fired utility boiler, a waste incinerator as well as from jet engines are summarized in Section II.4.3, Sensors for Advanced Combustion Systems, Global Climate & Energy Project, Stanford University, 2004, by Hanson et al. In Thompson et al., US Patent Application Publication US 2004/0191712 A1, a tunable near-infrared diode laser and absorption spectroscopy system to was applied to combustion applications in the steelmaking industry.
Kitchen, et al., U.S. Pat. No. 6,258,978, discloses a method of making vinyl acetate by contacting ethylene, acetic acid, and oxygen in the presence of a catalyst to produce an outlet stream. The concentration of oxygen in the outlet stream is maintained at or near its flammability limit. Kitchen points out that paramagnetic analyzers cannot be used where high temperature and pressure conditions are encountered, for example, adjacent to the reactor outlet.